Here's why:
* Past tense: The past tense of "lay" is laid, not "layed".
* Past participle: The past participle of "lay" is also laid, not "layed".
Therefore, it doesn't follow the usual pattern of adding "-ed" to the base form to form the past tense and past participle.
Example:
* Present: I lay the book on the table.
* Past: I laid the book on the table yesterday.
* Past participle: The book has been laid on the table.